It is a well-known fact that powder metal technique gives distinct advantages in producing near net shape products, i.e. products which give directly from powder, a finished product with minimum material and energy waste to a final cost lower than conventional production via forged, cast and/or machined parts. In many cases the properties of the powder metal product are superior.
Sintering of steel powder to full density is receiving an increasing interest due to the economics and energy savings in this process. This process usually requires high sintering temperatures unless the steel powder is mixed with a low melting additive, which acts like a glue for the more high melting powder. Two typical low melting additives are cupper and boron. However as these additions have detrimental effects on some of the properties for a steel product, for example welding or corrosion, these types of additions are prohibitive when fully dense parts from steel powder shall be produced meeting the requirements of wrought steel.
Another way to produce fully dense products via sintering is to use high temperatures in order to increase the sintering speeds and reach full density.
In the patent EP 1 047 518, it is shown that a high speed compaction process (HVC process) together with an agglomerated spherical metal powder offers distinct advantages.
In metal injection molding (MIM) an extremely fine powder, usually around 20 micron, is used giving the possibility to sinter to full density, due to the high surface activity of the fine and pure powders, which are usually gas atomized. These fine powders are very costly to produce and usually difficult or impossible to use for products with larger weight such as over for instance 50-100 grams.
Another way to produce fully dense products from powder with properties equal or better than wrought products is to use hot isostatic pressing (HIP) of a powder mass. The mass of powder must then be encapsulated in a “capsule”, i.e. a container which embeds the powder mass against the surrounding pressure medium, normally argon gas. The container normally used is made of a steel sheet. Practically and economically this makes the technique limited to relatively large parts, normally for instance 5 kg or more. There are also limitations regarding more complicated shapes due to the cost of capsule fabrication.
This means that there is an important product area ranging from approximately 50 grams up to about 5 kg which today for economical and practical reasons can't be efficiently targeted using the present state of the art.
One limitation when using compaction of metal powder, is that even if it is possible to get very high green densities, which is favorable in order to reach full density, for certain alloys the high temperature sintering can give problems due to formation of different phases or precipitations, which cannot be eliminated in later processes such as hardening, tempering or soft annealing, as there are no further breaking of such structures due to the near net shape product.
One area where there is room for improvement is that at high temperatures, especially when different structural phases occur critical grain growth can occur, i.e. big grains are formed which further impair the mechanical properties, especially impact properties and elongation. This is especially true when the material before sintering has been subjected to a small cold deformation. In such a case critical grain growth occurs easier.
Powder products, which have not reached full density cannot be hot isostatic pressed (hipped) without an enclosing container, because interconnecting porosity in the powder product makes the HIP operation useless. However, if the density of the pressed product is high enough approaching the full theoretical density, the pressed product can be hipped without a capsule and thereby reaching full density if the right parameters are used. This is usually done at a lower temperature than by high temperature sintering, thereby avoiding the above mentioned problems with precipitations and grain growth. As a rule of thumb green density over 95% TD gives a closed porosity and these products can therefore be hot isostatic pressed to full density without encapsulation.
Another room for improvement concerns the upper limit of densification. Due to the adiabatic effect, described in the patent EP 1 047 518, it is possible to reach very high densities, way over the conventional pressing technique. However, due to the need for debinding the binder such as a hydrocolloid it is necessary to stop the densification at a certain upper limit to allow the binder to evaporate during this step.
Other phenomena can also occur at extremely high densities with the binder incorporated, such as for instance blisters in the surface.
In the state of the art carbides accumulate and are preserved when the sintered body cools down after sintering at high temperature. These types of structures are impossible or very difficult to remove by subsequent heat treatment at lower temperatures, due to high content of carbide forming agents such as vanadium, tungsten, and chromium. In conventional production these types of structures are broken down in subsequent rolling, forging etc, when the cast structure is further processed to the final product of bars, sheet etc. Impact values are normally ranging between 50 to 150 joule, depending of hardness after hardening/tempering. However when the intention is to produce a net shape or near net shape without any or with only minor subsequent deformation process, this possibility to break down the defect structure does not exist.